Perpendicular recording has been developed in part to achieve higher recording density than is realized with longitudinal recording devices. A PMR write head typically has a main pole layer with a small surface area at an air bearing surface (ABS) and coils that conduct a current and generate a magnetic flux in the main pole that exits through a write pole tip and enters a magnetic media (disk) adjacent to the ABS. The flux may return through a shield structure to the back gap region which connects the main pole with the shield structure. There is typically one or more write shields on the write gap layer above the main pole and along the ABS and an upper section of the shield structure which may have an arched shape is formed over the coil layer and connects the one or more write shield sections along the ABS to the back gap region.
Perpendicular magnetic recording has become the mainstream technology for disk drive applications beyond 150 Gbit/in2. The demand for improved performance drives the need for a higher areal density which in turn calls for a continuous reduction in transducer size. A PMR head which combines the features of a single pole writer and a double layered media (magnetic disk) has a great advantage over LMR in providing higher write field, better read back signal, and potentially much higher areal density. Typically, today's magnetic head consists of a writer and a reader as separate elements that are formed adjacent to one another along an ABS. The read head may be based on a TMR element in which a tunnel barrier layer separates two ferromagnetic (FM) layers where a first FM layer has a fixed magnetization direction and the second FM layer has a magnetic moment that is free to rotate about a direction orthogonal to the direction of the magnetic moment in the reference “fixed” layer. The resistance across the barrier changes as the free layer moment is rotated. This signal is used to detect the small magnetic field from the recorded magnetization pattern on the media.
Reducing the magnetic spacing from read/write heads to the magnetic media during both writing and reading is the most important factor in achieving better performance in high density recording. The writer and reader are separated by several microns in a typical recording head and are made of several different materials each having a unique CTE. Therefore, the protrusion of the reader and writer are usually quite different due to the effect of varying operating temperatures, applying dynamic flying height (DFH) power to actuate the reader or writer, or from write current excitation. In addition, the point with minimum spacing to disk could be located away from either the reader or the writer, imposing further restrictions to achievable magnetic spacing during reading and writing. Improvements in PMR head design are needed to control the protrusion differences at the writer, the reader and the minimum point, and its variation. In particular, for the touch down and then back off mode of operation using DFH, if the writer protrusion is much more than the reader protrusion, then the minimum reader spacing is determined by the excess protrusion plus any initial protrusion. The ratio of reader protrusion rate/writer protrusion rate is called the gamma ratio. A lower gamma ratio means the writer protrusion rate is much higher than the reader protrusion rate, and could potentially put a greater limit to achievable reader spacing.
An important head design objective is to achieve a gamma ratio as close as possible to 1 which is ideal for tribology and magnetic performance since it keeps the gap between reader and writer at a constant value independent of the DFH power used for actuation. From a drive reliability point of view, the reader should not be at the minfly point which is the mechanically closest part of the head to the disk because the read sensor is more sensitive to mechanical impact. But the additional spacing margin for the reader needs to be kept to as small a number as reliability allows in order to have the best read back performance possible. The shield section above the coil layer (also known as a pp3 structure or third write shield) is known to provide a better gamma ratio when comprised of a lower CTE material such as a high moment CoFe alloy than when made from a lower moment material optimized for magnetic performance such as a 19 kG CoFeNi alloy. On the other hand, the high moment alloy has less desirable magnetic performance which leads to poorer write current saturation speed. Thus, neither a high moment material nor a low moment material by itself can provide an optimal performance. An improved pp3 design is desired in order to simultaneously achieve good mechanical performance (gamma ratio) and magnetic performance such as write current saturation speed.
A search of the prior art revealed the following references that relate to a write shield.
U.S. Patent Application No. 2007/0165330 that the write shield may be comprised of two or more layers and includes the entire shield structure on the write gap layer along the ABS. The composition and desired properties of the write shield structure are not disclosed.
U.S. Patent Application No. 2007/0279803 includes a write shield comprised of a lower throat defining layer on the write gap layer and a yoke structure made of FeNi, FeCo, or FeCoNi that is formed over the coil layer.
In U.S. Pat. No. 7,221,539, a main shield is described as having a composition including Co, Fe, CoNiFe, FeCo, and composites and laminates thereof, but there is no disclosure regarding the content of the upper shield section of the main shield structure.
U.S. Patent Application No. 2008/0002293 shows a write shield over a coil layer wherein the write shield is made of CoNiFe or CoFe.
U.S. Patent Application No. 2006/0002021 describes a write shield structure with three write shield layers including an upper (arched) shield having a CoNiFe composition.
U.S. Pat. No. 7,307,815 discloses an overarching yoke portion of a shield structure that is made of CoNiFe with a thickness between 0.5 and 1.5 microns.
None of the prior art references refer to modifying the composition of the pp3 portion of the write shield in order to optimize both magnetic and mechanical properties.